PREDICTABILITY OF THE ONSET OF BLOCKING AND STRONG ZONAL FLOW REGIMES

Flows with high and low sensitivity with respect to the initial condit ions for onset of blocking (BL) and strong zonal flow (SZF) regimes ha ve been analyzed. The author has considered BL and SZF regimes at 20 d egrees W (Atlantic region) and at 150 degrees W (Pacific region). The BL and SZF regimes are characterized by the same dipolelike anomaly pa ttern but with opposite signs. Experiments have been performed with a three-level quasigeostrophic model triangularly truncated at wavenumbe r 21 (T21QG) and its tangent linear and adjoint versions. The sensitiv ity is calculated by perturbing the reference flow with perturbations that optimally trigger the onset of a BL or SZF regime after a prescri bed forecast time. For forecast times larger than 3 days an iterative technique is used to take into account nonlinear growth of the perturb ations. The flows with a high sensitivity show an intensified jetstrea m to the west of a diffluent flow. The strong jetstream by itself resu lts in large perturbation growth. The presence of a diffluent flow amp lifies the growth and gives perturbations a typical dipolelike charact er. Idealized experiments with a barotropic T21 model confirm that the se properties increase the sensitivity. Sensitive flows are also chara cterized by an enhanced ridge upstream of the intensified jet stream. This does not directly influence the sensitivity but is associated wit h an intensified jet stream. The flow patterns of periods with low sen sitivity are more zonal and weaker. The diffluence of the Bow also res ults in an asymmetry between sensitivity for BL and SZF onset in the m edium range. Nonlinear feedback mechanisms increase sensitivity toward BL and decrease sensitivity toward SZF. Finally, it is shown that, on average, a transition toward BL corresponds with a larger than averag e sensitivity and that the sensitivity is larger when the transition i s stronger. Transitions toward an SZF correspond, on average, with an average sensitivity independent of the strength of the transition. The precursor patterns of transitions toward BL have similar characterist ics as the flows with a high sensitivity. Thus, blocking onset is like ly to be in many cases an inherently sensitive phenomenon.